12

LT1308A/LT1308B

1308abfa

when operating from a battery composed of alkaline

cells. The inrush current may cause sufficiency internal

voltage drop to trigger a low-battery indicator. A pro-

grammable soft-start can be implemented with 4 discrete

components. A 5V to 12V boost converter using the

When this current exceeds 0.7V/33k, or 21

μA, current

flows into the base of Q1. Q1’s collector then pulls

Δ

Δ

V

t

V

kC

07

33

4

.

•

waveforms for Figure 16’s circuit are pictured in Figure

17. Without the soft-start circuit implemented, the inrush

current reaches 3A. The circuit reaches final output

voltage in approximately 250

μs. Adding the soft-start

components reduces inductor current to less than 1A, as

detailed in Figure 18, while the time required to reach final

output voltage increases to about 15ms. C4 can be

adjusted to achieve any output slew rate desired.

SW

LT1308B

GND

FB

SHDN

+

330pF

100k

SHUTDOWN

SOFT-START

COMPONENTS

C1

47

μF

C4

33nF

Q1

R4

33k

R3

33k

5V

D1

L1

4.7

μH

12V

500mA

C2

10

μF

100pF

11.3k

47k

10k

C1: AVX TAJ476M010

C2: TAIYO YUDEN TMK432BJ106MM

D1: IR 10BQ015

L1: MURATA LQH6C4R7

Q1: 2N3904

1308 F16

Figure 16. 5V to 12V Boost Converter with Soft-Start Components Q1, C4, R3 and R4.

Soft-Start

In some cases it may be undesirable for the LT1308A/

LT1308B to operate at current limit during start-up, e.g.,

2V/DIV

500

μs/DIV

1308 F15

2A/DIV

5V/DIV

Figure 15. 5V SEPIC Start-Up from 9V Input into 10

Ω Load

1V/DIV

2A/DIV

5V/DIV

500

μs/DIV

1308 F14

Figure 14. 5V Boost Converter of Figure 1.

Start-Up from 3V Input into 5

Ω Load